Components of gas turbines are operated in a highly aggressive environment which can cause damage to the component in service. The environmental damage may occur in various forms in the hot combustion gas environment, such as particle erosion, different types of corrosion and oxidation, and complex combinations of these damage modes. The rate of environmental damage can be reduced by the use of protective layers.
For example it is known that chromium provides excellent protection against so called type I and type II hot corrosion. In this regard, diffusion coatings produced by the diffusion of chromium and aluminium into the alloy substrate have long been used to provide this protection. MCrAlY overlay coatings (where M is Ni or Co or a combination of the two) have been applied as an alternative to diffusion coatings at higher temperatures to protect against oxidation. Diffused chromium alone is known to provide excellent protection against relatively low temperature type II hot corrosion, and further to be strain tolerant.
Recent developments have shown that it is favourable to provide different types of coatings on different parts of a component. The coatings are chosen such that they are especially adapted to the thermal and corrosive conditions being present on the parts of the component during use.
U.S. Pat. No. 6,296,447 B1 discloses a gas turbine component with a location-dependent protective coating. The component is a turbine blade with a root, a neck, a platform, and an airfoil extending from the platform, having an outer and an inner surface defining cooling passages therethrough. A first coating is provided on at least a portion of the platform, a second coating is provided on the outer surface of the airfoil and a third coating is provided on the inner surface of the airfoil. The first coating differs in its composition from the second coating and the second coating differs in its composition from the third coating.